In the field of optical disc player, recently an optical head capable of accessing randomly an arbitrary position on a disc at high speed is attracting a wide attention.
The conditions of high speed access may be summarized below.
That is, there are two requirements for high speed access of the optical head. The first is that the speed of moving the optical head in the radial direction of the disc should be high, and the second is that the tracking servo should be stable.
To access at high speed, generally two processes--that is, rough access and fine access--are combined.
The rough access is a process of moving the optical head approximately to the radical position at a desired address position on the disc. The approximate radial position is known, for example, by attaching a linear potentiometer to the optical head and reading its voltage. Supposing the distance between the radial position before moving and that of the desired address track to be x, the fastest rough access is achieved by the optical head up to x/2 position at maximum acceleration and slowing down from x/2 to x position at maximum deceleration. It is ideal if the desired track is reached by the rough access alone, and in a magnetic disc or the like, the access is completed by this rough access alone. However, in the magnetic disc, the track width is about tens of .mu.m to hundreds of .mu.m, while the track width of an optical disc is as narrow as 1.6 .mu.m, and it is extremely difficult to reach the desired address track by the rough access alone. Accordingly, by reading the address of the track reached by rouch access, the light spot is caused to jump from this address track to the desired address track by a jumping pulse. This is called fine access.
Therefore, to access in the shortest time, it is necessary to quicken the moving speed of the optical head and bring it as closely to the desired track as possible in the rough access. In the fine access, it is required to jump over may tracks accurately in a short time, and for this purpose, the wide drawing range of the tracking servo and high servo gain are indispensable.
Various optical heads have been known conventionally, but, for example, to access within 0.1 second, sufficient characteristics are not obtained by either rough access or fine access. The main reason is that the moving part is heavy. Conventionally, the usual practice is to move the entire optical head by means of a linear motor, but since the optical head comprises a laser, a focusing actuator and its magnetic circuit, tracking actuator and its magnetic circuit, mirrors and a polarizing prism, lenses, a photo detector, a preamplifier,and other parts, its weight is at least several hundred grams. The rough access time T is expressed, assuming the moving part weight to be m, drive force to be F and moving distance to be x, as follows. EQU T=2.sqroot.mx/F
This equation is derived in the following sequence. Assuming acceleration for a moving distance of x/2 and decelerate from x/2 to x, the equation of motion EQU F=m(d.sup.2 x)/(dt.sup.2)
is integrated. In the prior art, supposing F=3N, m=400 g, and x=40 mm, for example, the rough access time is T=0.15 sec. Therefore, in the prior art, it takes as long as 0.15 sec for the rough access alone.
In the fine access, conventionally, the drawing range of tracking was too narrow. In a disc for recording and reproducing, the method of picking up an error signal of the tracking servo from the far-field pattern of diffracted light from the guide groove for tracking is called the far-field method, and in the conventional tracking method of swinging the objective lens in the radial direction of the disc or using a tracking mirror, the drawing range of the tracking servo is narrow.
For instance, in the method of swinging the lens for tracking purposes, as shown in FIG. 1(a), the reflected light from disc 3 moves along with the movement of the lens 2 which is driven by X-Y drive 1, and the movement of the reflected light is superposed as an offset on the error signal of the tracking of a fixed two-division photo detector 4, thereby narrowing the drawing range of tracking. This point may be improved, for example, by the arrangement as shown in FIG. 1(b). That is, when the photo detector 5 for detection of the tracking signal by the far-field method is integrally constructed with the lens 2, the drawing range is expanded about 2.5 times as compared with that of the method of swinging the lens 2 alone. (For example, this is described in detail in the Preprint of Lectures at the Applied Physics Congress, fall 1981, p. 121.) Even in this method, however, a drawing range wider than luminous flux diameter entering the lens 2 cannot be obtained, or a photo detector, a .lambda./4 plate and a polarizing prism must be assembled, together with the lens, into the actuator, which results in a heavy weight, a complicated structure, and difficulty in manufacture.
Another method has been proposed, as shown in FIG. 1(c), in which 2 is swung only the lens for focusing, a mirror 8 is fixed to a stand 7 which supports this lens 2 by means of a leaf spring 6, and this stand 7 is swung for tracking. This method is, for example, described in the Journal of Society of Electronic Communications (1983, No. 8, p. 838). In this method, however, although the stand 7 is swung by fixing the coil for tracking, since the light spot which is to be moved actually corresponds to the movement of the lens 2, it is as if the light spot is swung through the leaf spring 6 which supports the lens 2. Therefore, by the action of the leaf spring 6, secondary resonance appears in the tracking curve, so that the gain of the servo cannot be raised sufficiently. Or, since the circular gap in the magnetic circuit for focusing is directed upward and the gap in other magnetic circuit for tracking is directed sideways, the drawing range of tracking is limited to be within the gap in the magnetic circuit for focusing, and only a range of 1 to 2 mmm can be obtained. In this method, too, the entire optical head must be moved in for rough access.
Thus, in the conventional methods, the entire optical head must be moved in rough access for shortening the access time, and the heavy weight of this head has been a bottleneck for high speed access, while in the fine access there have also been serious problems, such as a narrow drawing range of tracking, heavy weight of moving parts, and insufficient gain of the tracking servo for achieving high speed access.